Rear outer discharge nozzle

ABSTRACT

A rear outer discharge nozzle for a gas turbine engine, the rear outer discharge nozzle forming a loop around an axis and comprising: a first part; a second part wrapped around the first part; and a radial slit, having first and second side walls; wherein the first part is shaped to provide the first side wall of the radial slit, and the second part is shaped to provide the second side wall of the radial slit; and wherein the second part comprises a break, splitting the second part in the axial part.

The present disclosure concerns a rear outer discharge nozzle (RODN) for a gas turbine engine. In particular the RODN has a radial slit, or birdmouth, that can be formed in situ during assembly of the engine.

The rear outer discharge nozzle (RODN) is a transition part between the combustion liner and the high pressure nozzle guide vanes (HPNGVs) in a gas turbine engine. The RODN is conventionally assembled onto the high pressure (HP) turbine module using a process similar to the way in which a bicycle tyre is assembled onto a wheel rim. A radial slot or “birdmouth” in the RODN is gradually fitted into place, moving circumferentially around the RODN. Dis-assembly can be performed by reversing the process.

However, the conventional RODN design has, in a few known instances, become disengaged from the HPNGV. This can result in leakage of cooling air, by increasing the axial gap in front of the HPNGV platform. It can also cause the HPNGV outer chordal seal to lift creating further leakage of cooling.

Further, experienced assemblers and technique are required to ensure the RODN is not damaged or deformed during the assembly/dis-assembly process described above. In particular, the manual assembly process to accommodate the HPNGVs radial rails into the radial birdmouth, if done incorrectly, can cause the inner/outer radial birdmouths (i.e. the sides forming the radial slit of the birdmouth) to permanently deform. Even if the assembly is completed, the deformation of the radial birdmouth during operating conditions might lead to dis-engagement of HPNGVs radial rails. Dis-engagement leads to wear and cooling air leakages. Even if there is no leakage, wear is not repairable and so the entire RODN must be scrapped.

The present invention aims to mitigate these issues by providing a RODN design that mitigates against damage during assembly and which can minimise waste during repair.

According to a first aspect there is provided a rear outer discharge nozzle for a gas turbine engine, the rear outer discharge nozzle forming a loop around an axis and comprising one or more of: a first part; a second part wrapped around the first part; a radial slit, having first and second side walls; wherein the first part is shaped to provide the first side wall of the radial slit, and the second part is shaped to provide the second side wall of the radial slit; wherein the second part comprises a break, splitting the second part along the axial direction. By providing separate first and second parts, the second part can be assembled around the first part after the first part has been positioned in an engine. The break in the second part means it is not a complete loop, but a piece with two ends that can be joined together to form a loop around the first part. This allows the second part to be positioned as required, after the first part is already in situ in an engine. The separate first and second parts also mean that the parts can be replaced individually.

Optionally, the second part comprises fastening points either side of the break. These can attach the ends of the second part together, thereby fixing the second part in place, clamping it around the first part.

Optionally, the fastening points are attached to the second part, optionally by welding. Alternatively, the fastening points can be integrally formed with the second part.

Optionally, the second part is fastened to the first part, optionally by one or more bolts. This can fix the size of the radial slit.

Optionally the first and second part engage to inhibit angular rotation of the second part with respect to the first part around the axis. This can prevent wear of the two parts, and ensures that a good air flow can be maintained by avoiding the air holes through the RODN becoming blocked.

In one way of inhibiting angular rotation, the first part comprises a projection, and the second part comprises a slot that fits around the projection.

In another way of inhibiting angular rotation, the first part comprises a slot, and the second part comprises a projection that fits within the slot.

In another way of inhibiting angular rotation, the first part comprises a projection that fits within the break.

In another way of inhibiting angular rotation, an anti-rotation part is configured to inhibit angular rotation of the second part with respect to the first part around the axis. The first part can comprise an orifice and the second part comprise another orifice, and the anti-rotation part is a fastening element attached to the second part and passing through the orifice in the first part and the orifice in the second part. Additionally or alternatively, the first part can comprise a first slot, the second part can comprise a second slot aligned with the first slot, and wherein the anti-rotation part is a locking plate positioned within the first and second slot to thereby lock the relative positions of the first and second parts.

According to another aspect of the invention, there is provided a gas turbine engine comprising a rear outer discharge nozzle according to any one of the preceding arrangements.

According to another aspect of the invention, there is provided a method of assembling a gas turbine engine, the method comprising one or more of: positioning a first part of a rear outer discharge nozzle so as to form a first side wall of a radial slit around radial rails of high pressure nozzle guide vanes in the gas turbine engine; positioning a second part of a rear outer discharge nozzle to wrap around the first part and to form a second side wall of the radial slit; and fastening the first part to the second part, to thereby attach the rear outer discharge nozzle around the high pressure nozzle guide vanes.

According to another aspect of the invention, there is provided a gas turbine engine combustion arrangement comprising a combustion chamber, a rear outer discharge nozzle and an array of turbine nozzle guide vanes, the combustion chamber comprising an outer combustion liner, the outer combustion liner of the annular combustion chamber having a downstream end, the array of turbine nozzle guide vanes having outer platforms, the outer platforms of the turbine nozzle guide vanes having radial rails, the array of turbine nozzle guide vanes being located downstream of the combustion chamber, the rear outer discharge nozzle being located between the downstream end of the outer combustion liner of the combustion chamber and the array of turbine nozzle guide vanes, the rear outer discharge nozzle having an annular axially extending slot, the downstream end of the outer combustor liner of the combustion chamber locating in the annular axially extending slot in the rear outer discharge nozzle, the rear outer discharge nozzle having an annular radially extending slot, the annular radially extending slot having first and second side walls, the radial rails on the outer platforms of the turbine nozzle guide vanes locating in the annular radially extending slot in the rear outer discharge nozzle, the rear outer discharge nozzle forming a loop around an axis and comprising: a first part having the annular axially extending slot; a second part wrapped around the first part; and the first part is shaped to provide the first side wall of the annular radially extending slot, and the second part is shaped to provide the second side wall of the annular radially extending slot; wherein the second part comprises a break, splitting the second part along an axial direction such that the second part is not a complete loop, the second part has two ends separated by the break; and the second part is fastened to the first part.

The two ends of the second part may comprise fastening points and the fastening points of the second part are removably fastened together.

The second part may comprise two or more breaks splitting the second part along an axial direction into two or more pieces.

The skilled person will appreciate that except where mutually exclusive, a feature described in relation to any one of the above aspects may be applied mutatis mutandis to any other aspect. Furthermore except where mutually exclusive any feature described herein may be applied to any aspect and/or combined with any other feature described herein.

Embodiments will now be described by way of example only, with reference to the Figures, in which:

FIG. 1 is a sectional side view of a gas turbine engine;

FIG. 2 shows in more detail the circled region labelled R in FIG. 1;

FIG. 3 shows a conventional rear outer discharge nozzle (RODN);

FIG. 4 shows a RODN having a multi-part design;

FIGS. 5A and 5B illustrate a break in the outer radial birdmouth layer of the RODN of FIG. 4;

FIGS. 6A and 6B illustrate an alternative support plate to that illustrated in connection with FIGS. 5A and 5B;

FIGS. 7A-7C illustrate an alternative multi-part RODN construction;

FIGS. 8A and 8B illustrate a combination of the multi-part RODN design of FIGS. 5A and 5B with that of FIGS. 7A-7C;

FIGS. 9A and 9B illustrate an anti-rotation washer for use with a multi-part RODN;

FIGS. 10A-10C illustrate a slot and tab anti-rotation arrangement for a multi-part RODN;

FIGS. 11A-11E illustrate a locking plate anti-rotation arrangement for a multi-part RODN;

FIGS. 12A and 12B illustrate a anti-rotation tab formed as part of the outer radial birdmouth layer of a multi-part RODN; and

FIGS. 13A-13F illustrate anti-rotation tabs for projecting into a break in an outer radial birdmouth layer in a multi-part RODN.

With reference to FIG. 1, a gas turbine engine is generally indicated at 10, having a principal and rotational axis 11. The engine 10 comprises, in axial flow series, an air intake 12, a propulsive fan 13, an intermediate pressure compressor 14, a high-pressure compressor 15, combustion equipment 16, a high-pressure turbine 17, an intermediate pressure turbine 18, a low-pressure turbine 19 and an exhaust nozzle 20. A nacelle 21 generally surrounds the engine 10 and defines both the intake 12 and the exhaust nozzle 20.

The gas turbine engine 10 works in the conventional manner so that air entering the intake 12 is accelerated by the fan 13 to produce two air flows: a first air flow into the intermediate pressure compressor 14 and a second air flow which passes through a bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 14 compresses the air flow directed into it before delivering that air to the high pressure compressor 15 where further compression takes place.

The compressed air exhausted from the high-pressure compressor 15 is directed into the combustion equipment 16 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 17, 18, 19 before being exhausted through the nozzle 20 to provide additional propulsive thrust. The high 17, intermediate 18 and low 19 pressure turbines drive respectively the high pressure compressor 15, intermediate pressure compressor 14 and fan 13, each by suitable interconnecting shaft.

Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. By way of example such engines may have an alternative number of interconnecting shafts (e.g. two) and/or an alternative number of compressors and/or turbines. Further the engine may comprise a gearbox provided in the drive train from a turbine to a compressor and/or fan.

FIG. 2 shows in more detail the circled region labelled R in FIG. 1, between the combustor 16 and the turbine nozzle guide vanes (NGVs) 24 and turbine blades 25 of the high pressure (HP) turbine 17. A rear inner discharge nozzle (RIDN) sealing ring 26 extends across the gap between an inner end-wall 28 of the combustor and NGV segment inner platforms 29, and a rear outer discharge nozzle (RODN) sealing ring 27 extends across the gap between an outer end-wall or liner 30 of the combustor and NGV segment outer platforms 31. The RIDN 26 and the RODN 27 take up the relative axial and radial movement between the combustor and the NGVs.

FIG. 3 shows a typical RODN 27 design in more detail. As mentioned above, the RODN 27 is a part of the combustion module 16 and interfaces with the combustion liner 30 and the static High Pressure Nozzle Guide Vanes (HPNGV) 24. As such, the RODN 27 fulfils various functions. It acts as transition part between the combustion liner 30 and the HPNGV 24. It provides a slot 275 in the axial direction to accommodate axial thermal growth of the combustor liner 30 and also helps the liner 30 to maintain concentricity with the HPNGV 24. Air holes 276, 277 through the layers of the RODN 27 allow uninterrupted flow of cooling air to the holes on the HPNGV 24 which control the combustor exit temperature and cooling the HPNGV 24. The RODN 27 also minimises uncontrolled flow of cooling air ahead of the HPNGV 24 outer platform leading edge.

As mentioned above, the RODN 27 interfaces with the combustor liner 30 via an axial slot, also referred to as the axial birdmouth 275. This is conventionally formed as a single piece. The interface with the HPNGV 24 is via a birdmouth 280 with a radial slot. Conventionally, the sides of the slot are referred to as the outer radial birdmouth 282 and inner radial birdmouth 281 (“inner” and “outer” being with respect to the overall axial length of the RODN 27, with the outer radial birdmouth 282 being axially furthest from the axial birdmouth 275 provided at the opposite side of the RODN 27). The inner and outer birdmouths 281, 282 are conventionally formed by separate pieces that have been permanently joined during the construction of the RODN 27, but before it assembled to the rest of the gas turbine engine 10.

The axial birdmouth 275 interfaces with combustor liner 30 at the aft end of the RODN 27. It allows relative axial movement, accommodates combustor radial distortions during operation, damps vibration & locates the RODN 27 to the liner 30 aft during assembly. The axial birdmouth part 275 can be either forged or flash welded, for example. The axial slot of the birdmouth 275 can be machined and can then be heat treated.

The radial birdmouth 280, comprising the slit formed by inner and outer radial birdmouths 281, 282, locates the RODN 27 axially during running, and interfaces with the outer platform front rail of HPNGV 24. This allows free radial movement relative to the HPNGV 24, and thus allows unrestricted thermal growth of the HPNGVs 24. The outer and inner radial birdmouths 281, 282 are shaped from layers of sheet metal, which are conventionally flash welded to form continuous 360° parts. Both inner and outer radial birdmouths 281, 282 are conventionally fusion welded together and finally heat treated, before the RODN 27 assembled to the rest of the gas turbine engine 10.

As mentioned above, the large air holes 276, 277 in the layers of the RODN 27 allow uninterrupted flow of cooling air to the holes on the HPNGV 24 for controlling the radial traverse and cooling the HPNGV 24. These holes are formed as a first set of holes 276 passing through the layer of material forming the inner radial birdmouth 281 and a second set of holes 277 passing through the layer of material forming the outer radial birdmouth 282. The holes 276 and 277 align to form a set holes passing through the RODN 27. As an example, circular holes in the shape of a race track (i.e. with two substantially parallel sides and two substantially semi-circular ends) can be spaced circumferentially around the RODN 27 air holes. The holes can be produced using conventional machining techniques.

As already described, the RODN 27 is conventionally assembled onto the high pressure (HP) turbine module 17 using a process similar to the way in which a bicycle tyre is assembled onto a wheel rim, but this can lead to damage of the inner/outer radial birdmouths 281, 282.

FIG. 4 illustrates a RODN 27 that is designed to ease assembly and reduce the risk of damage to the RODN 27 as it is fitted.

The RODN 27 of FIG. 4 has an axial birdmouth 275 and a radial birdmouth 280 as in the conventional designs. However, the radial slit of the radial birdmouth 280 is defined by mechanically separate parts that are shaped to provide the first and second sidewalls 281, 282, as opposed to parts that are welded together as in the conventional case.

The RODN 27 of FIG. 4, has a two-piece design. There is a first part 271 and a second part 272. The first part 271 comprises the axial birdmouth 275, and the layer of material 274 forming the inner radial birdmouth 281, i.e. the layer forming the first sidewall of the radial slit of the radial birdmouth 280. In other words, the first part 271 is shaped to provide the first sidewall 281 of the radial slit of the birdmouth 280. The inner radial birdmouth layer 274 is also provided with air-holes 276, in a conventional manner.

The first part 271 also includes a forward outer ring layer 273, which is discussed in further detail below.

The different elements of the first part 271 may be permanently joined together, e.g. by welding, in the usual way.

The second part 272 is a layer of material that is shaped to provide the outer radial birdmouth 282, i.e. the layer forming the second sidewall of the radial slit of the radial birdmouth 280. The second part 272 is also provided with air-holes 277, corresponding to the air-holes 276 in the inner radial birdmouth layer. The second part 272 wraps around the first part 271, but is not permanently joined to the first part 271. That is, in contrast to a conventional construction in which the layer forming the outer radial birdmouth 282 would be welded to the layer forming the inner radial birdmouth 281, there is no such welding in the arrangement of FIG. 4. Instead, the second part 272 can be fastened in position as discussed in more detail below.

One element of such fastening, as shown in FIG. 4, is the option of providing an interlocking feature 278, by which the second part 272 can interlock with the forward outer ring 273. In FIG. 4 this is achieved by the second part 272 locating over the rim of the forward outer ring 273. This can help position the second part 272 in the correct arrangement with respect to the first part 271, to correctly create the radial birdmouth 280.

Although not shown in FIG. 4 (but illustrated in FIGS. 5A and 5B), the second part 272 contains at least one split or break 290. As such, instead of the second part 272 being formed as a single continuous ring, the second part 272 has at least one break 290 that allows the ring of the second part 272 to be opened out. As such, when assembling the gas turbine engine 10, a different procedure can be followed for the attachment of the RODN 27. First, the first part 271 (being the subassembly of the axial birdmouth, inner axial birdmouth 275, inner radial birdmouth layer 274 and forward outer ring 273) can be positioned in place with respect to the combustion liner 30. Thereafter, the second part 272 can be fitted to provide the outer radial birdmouth 282 over the HPNGV 24 radial rails. The second part 272 can be located to fit with the interlocking feature 278, and then fastened in place.

In other words, in this procedure, the radial birdmouth 280 is constructed in situ, around the HPNGV 24 radial rails, rather than being formed prior to the assembly to the HPNGV 24 radial rails. The in situ construction reduces the possibility of damaging the birdmouth 280, compared to the conventional design, because it is not necessary to perform the conventional construction procedure discussed above, which can put stress on the HPNGV 24 radial rails and the inner and outer birdmouths 281, 282.

Another advantage of the multi-part approach to the design of the RODN 27 is that it provides an option for increasing the length of the radial birdmouth 280 that fits over the HPNGV 24 radial rails. Previous assembly techniques limit the overlap that can be present between the radial birdmouth 280 and the HPNGV 24 radial rails, because increasing the overlap means it becomes impossible to assemble the RODN 27 around the HPNGV 24 radial rails with the conventional technique. In the presently described approach, the radial birdmouth 280 is formed around the HPNGV radial rails as the second part 272 is fixed around the first part 271, and therefore the height of the radial birdmouth 280 can be increased. This in turn mitigates against RODN 27 disengagement.

As shown in FIG. 5A, the free ends 291 of the second part 272, either side of the break 290, can be fastened together. Such fastening can be achieved by providing fastening points 300 at each free end 291. The fastening points can be joined together using bolts 310 and washers 320 to strengthen the fastening points, if required, as shown in FIG. 5B. By passing bolts 310 through a fastening point 300 provided on either side of the break 290, and then attaching nuts to the ends of the bolts, the second part 272 can be joined to form a continuous loop. However, the option remains to remove the nuts and then the fastening bolts 310 and therefore remove the second part 272. This allows for the RODN 27 to be deconstructed, if necessary. It further allows for the first part 271 or the second part 272 to be replaced independently of each other, thereby minimising waste when a replacement is necessitated by wear for example.

In FIGS. 5A and 5B, the fastening points 300 are provided as an integral support strip that can be formed as part of the layer 272 that forms the outer radial birdmouth 282. However, the fastening points could be provided as a separate support strip 301, as shown in FIGS. 6A and 6B. FIG. 6A shows a perspective view of a single end 291 with a support strip 301 that is formed separately from the layer of material that forms the outer radial birdmouth 282. The support strip 301 is subsequently joined to that layer through resistance welding or some other permanent joining mechanism, e.g. at points 302. The support strips 301 can then function in exactly the same way as the support strips 300, as a fastening point. This is illustrated in FIG. 6B which shows two ends with support strip 301 meeting, to be bolted together (bolts not shown).

FIGS. 5A, 5B, 6A, and 6B illustrate a single break 290 in the second part 272. This results in the second part 272 being a single continuous piece, the ends of which 291 can be joined to form a loop. However, the second part 272 may be provided as multiple pieces, which can be joined together, in a similar way to that illustrated in FIGS. 5A and 5B or FIGS. 6A and 6B. The multiple pieces can then be fastened together to provide the loop of the second part 272. In that case, there will be multiple breaks 290 in the second part 272. That is, there can be two or more breaks 290, resulting in two or more pieces of the second part 272.

FIGS. 5A, 5B, 6A, and 6B illustrate the second part 272 being fastened to itself to position it in place. That is, wrapping the second part 272 around the first part 271 and then fastening the ends (or individual pieces) of the second part 272 to each other effectively clamps the second part 272 around the first part 271. In addition, the presence of a feature such as the interlocking feature 278 can help ensure that the second part 272 is kept in position with respect to the first part 271.

An alternative approach is shown in FIGS. 7A-7C. Again, although FIGS. 7A-7C only illustrates a single break 290, the approach is also applicable to second parts 272 constructed from multiple pieces. As shown in FIG. 7A, which is a sectional view through a RODN 27, the construction of the RODN 27 slightly differs from that in FIG. 4. FIG. 7B shows a perspective view of a section through the RODN 27, from the opposite direction to FIG. 7A. The outer layer 272 is directly bolted to the first part 271 with one or more bolts 330. As shown in FIGS. 7A and 7B, the bolt 330 may pass through both the forward outer ring 274 and the layer 273 forming the inner radial birdmouth. However, in other constructions, the layer 273 forming the inner radial birdmouth may be provided in a fashion more similar to that shown in FIG. 4, such that the second part 272 is only directly bolted to the forward outer ring 274.

As shown in the perspective view FIG. 7C, the direct attachment of the second part 272 to the first part 271 via one or more bolts 330 means that the ends 291 around a break 290 do not need to be directly fastened to each other. Instead, the fastening of the second part 272 to the first part 271 keeps the second part 272 in the correct position. There may be several bolts 330 provided around the circumference of the RODN 27. For example, there may be 10-12 bolts 330, or even more depending on the design requirements and the number of pieces forming the second part 272.

An advantage of the second part 272 being formed as a single piece (i.e. only having one break 290) is that there are fewer pieces to install, which can lead to simpler and quicker assembly. Alternatively, an advantage of having multiple pieces forming the second part 272 is that the pieces may undergo less deformation as they are positioned (as it may still be necessary to flex a single-piece second part 272 as it is wrapped around the first part 271—however, even in that case, the inner radial birdmouth undergoes no deformation).

FIGS. 8A and 8B illustrates that the approach shown in FIGS. 7A-7C can be combined with the approaches of FIGS. 5A, 5B, 6A, and 6B. That is, the second part 272 can be fastened to the first part 271, for example using bolts 330, whilst the ends 291 of the second part 272 can be fastened to each other, for example using bolts 310. FIG. 8B shows a perspective view, whilst FIG. 8B shows a cutaway view to reveal the underlying features. One reason the combined approach of FIGS. 8A and 8B might be advantageous is that the fastening prevents the second part 272 rotating with respect to the first part 271, which would cause additional wear (and potentially cause blockages if the air holes 276, 277 became misaligned).

FIGS. 9A-13F illustrate alternative anti-rotation mechanisms that may be used separately or in combination with each other and/or the fastening of the second part 272 to the first part 271.

FIGS. 9A and 9B relates to the provision of an additional anti-rotation part 321, which is shown separately in FIG. 9A, and in use in FIG. 9B. Anti-rotation part 321 is a washer similar to washer 320 described with respect to FIGS. 5A and 5B. However, in addition to the main body of the washer there is a U-shaped hook 321 a extending from the main body. In use, the washer 321 can be assembled to a support strip 300 in a similar way to that discussed in connection with FIGS. 5A and 5B, and bolted thereto with bolts 310 and nuts associated with the bolts. However, when positioning the anti-rotation washer 321, it is first placed with the hook 321 a extending through the air-hole 277 through the second part 272, and also through an air-hole 276 through the first part 271. As the washer 321 is directly attached to the second part 272, the second part cannot move with respect to the hook 321 a extending through air hole 277. Moreover, because the hook 321 a extends around the edge of the lower air-hole 277, the first part 271 is prevented from rotating through hook. By providing a washer 321 on both sides of the break 290, with the hook 321 a extending in the opposite direction to the first anti-rotation washer 321, rotation in both directions is mitigated. Of course, the ‘hook’ 321 a need not be hook shaped—all that is required to provide a rotation barriers is that it extends into, and by the edge of, the air hole 277.

FIGS. 10A-10C illustrates an anti-rotation approach that is based upon the first part 271 and second part 272 engaging in a manner that inhibits angular rotation of the second part with respect to the first part. As shown in FIG. 10A, the first part 271, in this case the forward outer ring 273, can be provided with a projection or tab 341, which fits in a corresponding slot 342 in the second part 272, illustrated in FIG. 10B. FIG. 10C shows the two parts fitted together.

In the FIGS. 10A-10C examples, the tab 341 is provided on the rim of the forward outer ring, with the slot 342 in the edge of the second part 272. Such an arrangement is simple to manufacture. However, the tab 341 could be provided in a different location, or connected to a different element of the first part 271. Similarly, the slot 342 could be provided in the main body of the second element 272. What is relevant is that the projection 341 and slot 342 can be aligned during the assembly of the second part 272 to the first part 271, as shown in FIG. 10C. Because the slot 342 fits around the tab 341, the second part 272 is prevented from rotating with respect to the first part 271.

The projection 341 and slot 342 arrangement illustrated in FIGS. 10A-10C can be repeated at various points around the RODN 27. For example, a projection 341 and slot 342 may be provided at opposite points (i.e. separated by 180° around the circumference) of the RODN 27. Alternatively, where the second part 272 of the RODN 27 is formed from several pieces, at least one slot and tab arrangement may be present on each piece. In some arrangements, a slot and tab arrangement can be provided midway between any breaks 290.

FIGS. 11A-11E shows an alternative anti-rotation arrangement in which a slot 343 is provided in the first part 271 (again illustrated as the forward outer ring 273) rather than a projection 341 (as in the FIGS. 10A-10C arrangement).

As shown in FIG. 11A, a slot can be provided in the first part 271, for example in the rim of the forward outer ring 273. A corresponding slot 344 can be provided in the second part 272 as shown in FIG. 11B. The slot 344 is provided between the end edge 291 of the second part 272 and the support plate 300 which is used for fastening the ends 291 of the second part 272 to each other.

By aligning the slot 344 in the second part 272 with the slot 343 in the first part 271, an anti-rotation blocking plate can be positioned that passes through both slots 343, 344. This is illustrated in FIG. 11C, and the anti-rotation locking plate 322 is shown in FIG. 11D. In this example, the anti-rotation locking plate 322 is a variation of the washer 320, previously discussed in connection with FIGS. 5A and 5B. The washer 322 acts as a washer for the bolt 310 connecting the ends 291 of the second part 272 to each other via the support plates 300. In the FIGS. 11A-11E arrangement, the washer 322 comprises an extension 345 that projects through the slot 344 and into the slot 343. As a result, the washer 322 also acts as the anti-rotation locking plate. In other words, the presence of the washer 322 mitigates against any rotation between the first part 271 and the second part 272, because such relative movement is constrained by the washer 322 being present within both slots 343 and 344. As in the FIGS. 9A and 9B anti-rotation arrangement, the modified washer 322 can be provided on either side of the break 290 and this is illustrated in FIG. 11E.

As an alternative to providing an entirely separate locking element, as in the arrangement of FIGS. 11A-11E, FIGS. 12A and 12B illustrates how a similar effect can be achieved by providing a specific anti-rotation tab 346 as part of the second part 272. In other words, the tab 346 can be integrally formed as part of the second part 272, or permanently attached thereto. The tab 346 is shown in FIG. 12B, and is shown in location in FIG. 12A. The tab 346 would replace the need for the locking plate 322 in the FIGS. 11A-11E arrangement. Instead, in use, the tab 346 would engage with the slot 343 of the first part 271 by acting as a projection which fits into the slot 343.

FIGS. 13A-13F shows alternatives in which anti-rotation tabs may be provided as part of the first part 271. FIG. 13A illustrates an anti-rotation tab 347 similar to 346 in FIGS. 12A and 12B. However, the anti-rotation tab 347 is attached to the first part 271 (as illustrated, the forward outer ring 273), rather than the second part 272. In use, as shown in FIG. 13C, the anti-rotation tab 347 acts as a projection that fits within the break 290 in the second part 272. By fitting within the break 290, the second part 272 is unable to rotate past the anti-rotation tab 347. FIGS. 13D-F illustrate a similar arrangement, with a differently shaped anti-rotation tab 348. Whereas anti-rotation tab 347 merely fits within the break 290, the anti-rotation tab 348 has a hook shape that fits back over the second part 272. This can provide an additional restraint to movement.

It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein. 

1. A rear outer discharge nozzle for a gas turbine engine, the rear outer discharge nozzle forming a loop around an axis and comprising: a first part; a second part wrapped around the first part; and a radial slit, having first and second side walls; wherein the first part is shaped to provide the first side wall of the radial slit, and the second part is shaped to provide the second side wall of the radial slit; and wherein the second part comprises a break, splitting the second part along the axial direction.
 2. A rear outer discharge nozzle according to claim 1, wherein the second part comprises fastening points either side of the break.
 3. A rear outer discharge nozzle according to claim 2, wherein the fastening points are attached to the second part.
 4. A rear outer discharge nozzle according to claim 2, wherein the fastening points are integrally formed with the second part.
 5. A rear outer discharge nozzle according to claim 1, wherein the second part is fastened to the first part.
 6. A rear outer discharge nozzle according to claim 1, wherein the first and second part engage, to inhibit angular rotation of the second part with respect to the first part around the axis.
 7. A rear outer discharge nozzle according to claim 6, wherein the first part comprises a projection, and the second part comprises a slot that fits around the projection.
 8. A rear outer discharge nozzle according to claim 6, wherein the first part comprises a slot, and the second part comprises a projection that fits within the slot.
 9. A rear outer discharge nozzle according to claim 6, wherein the first part comprises a projection that fits within the break.
 10. A rear outer discharge nozzle according to claim 1, further comprising an anti-rotation part configured to inhibit angular rotation of the second part with respect to the first part around the axis.
 11. A rear outer discharge nozzle according to claim 10, wherein the first part comprises an orifice and the second part comprises another orifice, and anti-rotation part is a fastening element attached to the second part and passing through the orifice in the first part and the orifice in the second part.
 12. A rear outer discharge nozzle according to claim 10, wherein the first part comprises a first slot, the second part comprises a second slot aligned with the first slot, and wherein the anti-rotation part is a locking plate positioned within the first and second slot to thereby lock the relative positions of the first and second parts.
 13. A gas turbine engine comprising a rear outer discharge nozzle according to claim
 1. 14. A method of assembling a gas turbine engine, the method comprising: positioning a first part of a rear outer discharge nozzle so as to form a first side wall of a radial slit around radial rails of high pressure nozzle guide vanes in the gas turbine engine; positioning a second part of a rear outer discharge nozzle to wrap around the first part and to form a second side wall of the radial slit; and fastening the first part to the second part, to thereby attach the rear outer discharge nozzle around the high pressure nozzle guide vanes.
 15. A method of assembling a gas turbine engine according to claim 14, wherein the second part comprises two or more pieces.
 16. A gas turbine engine combustion arrangement comprising a combustion chamber, a rear outer discharge nozzle and an array of turbine nozzle guide vanes, the combustion chamber comprising an outer combustion liner, the outer combustion liner of the annular combustion chamber having a downstream end, the array of turbine nozzle guide vanes having outer platforms, the outer platforms of the turbine nozzle guide vanes having radial rails, the array of turbine nozzle guide vanes being located downstream of the combustion chamber, the rear outer discharge nozzle being located between the downstream end of the outer combustion liner of the combustion chamber and the array of turbine nozzle guide vanes, the rear outer discharge nozzle having an annular axially extending slot, the downstream end of the outer combustor liner of the combustion chamber locating in the annular axially extending slot in the rear outer discharge nozzle, the rear outer discharge nozzle having an annular radially extending slot, the annular radially extending slot having first and second side walls, the radial rails on the outer platforms of the turbine nozzle guide vanes locating in the annular radially extending slot in the rear outer discharge nozzle, the rear outer discharge nozzle forming a loop around an axis and comprising: a first part having the annular axially extending slot; a second part wrapped around the first part; and the first part is shaped to provide the first side wall of the annular radially extending slot, and the second part is shaped to provide the second side wall of the annular radially extending slot; wherein the second part comprises a break, splitting the second part along an axial direction such that the second part is not a complete loop, the second part has two ends separated by the break; and the second part is fastened to the first part.
 17. A gas turbine engine combustion arrangement as claimed in claim 16, wherein the two ends of the second part comprise fastening points and the fastening points of the second part are removably fastened together.
 18. A gas turbine engine combustion arrangement as claimed in claim 16, wherein the second part comprises two or more breaks splitting the second part along an axial direction into two or more pieces. 